Why would the electrode on a spark plug get smashed

[WinnerCougar74]

I installed conventional plugs on a 1974 135hp Chrysler and while running at 3/4 throttle for about 10 minutes the motor stopped running well. I pulled the plugs and found the #3 plug electrode was smashed. No debris in the cylinder and 130psi compression just like the other 3 cylinders. Any idea what would cause this?

 

[GA_Boater]

Didn't this same thing happen before?

 

[Redbarron%%]

Piston hitting the electrode caused this.
Gasket squeezed too flat?
Rods stretched?
Block or head surfaced?
Reed valve passing over the piston?

 

[Chris1956]

Bad connecting rod bearing?

 

[foodfisher]

Wrong plug?

 

[racerone]

Judging by the color of the threads that plug is too long !

 

[Sunburn71]

Just a guess but seems like wrong plug. NGK might be the wrong application for that motor.

 

[racerone]

Check length of plug.------Brand does not matter.

 

[Sunburn71]

Check length of plug.------Brand does not matter.

I did not mean to imply that NGK was an improper BRAND just perhaps an application issue. Plug seems long to me.

 

[racerone]

Are / were all 4 plugs the same on this motor ?-----Length matters !-----This should be easy to solve.

 

[WinnerCougar74]

Thanks for the input. Yes this happened awhile back but I wanted to get more input on what could have caused it.

A few weird things, all the plugs are identical but only this one got smashed, It ran good for a few hours before this happened. there does not seem to be any play in the piston. When you turn the flywheel there is zero delay in the piston moving. If any of the piston parts where bad there would be a delay in the movement of the piston while turning the flywheel. #3 cylinder movement seems the same as the other pistons. The thread length on the spark plugs matches the thread length on the gapless plugs but the gapless don't have the electrode sticking out. Only thing I can come up with was #3 cylinder was running lean and getting hot. That made the electrode get very hot and the pressures in the cylinder caused the electrode to smash.

I guess for now I'm solving it with Champion L20V gapless plugs but I idle so much while searching for fish on the fish finder that I would prefer using the plugs with the electrode. Maybe I could find plugs with shorter threads.

 

[topgun3690]

I use the L20V in mine....they will foul a little bit when trolling around but clear up with a good full throttle run. If your plugs are fouling that bad maybe running a little too rich....idk.

 

[WinnerCougar74]

I like the look of the L20V. The bevel edge looks like it will clear fouling better. They are discontinued. Not sure if there is a comparable Champion. It's hard to know if these motors are running rich or lean. The only adjustment is the idle screw. I think a 1/8 turn can make the difference between fouling or sneezing and dying. My motor tends to sneeze real easy if it's even a little lean. Oh well it's better than thousands of dollars for an updated motor. Incidentally if I did upgrade I would go with an 80's Evinrude. A rebuilt 80's Evinrude is about $5000

 

[martymcfly62]

Piston hitting the electrode caused this.
Gasket squeezed too flat?
Rods stretched?
Block or head surfaced?
Reed valve passing over the piston?

Either of these is possible. Time to get out the micrometer!

 

[Sunburn71]

I had another thought on this, if the spark plug seat was machined a little too deep in the cylinder head, it would cause the plug to extend too far into the combustion chamber. Could do a measurement to compare to the other plugs at the top (plug wire end) to see if it's the same.

 

[440roadrunner]

You need to find out what REACH plugs are designed for that engine, IE go to the manufactuer's original info, shop manual, owners manual, wherever is specifies a plug, and then interchange that to what you have. I have lost all track of most plug numbering systems, because some of them have changed. In my day it was Autolite, AC or Champion.

Plugs are commonly 3/8 reach or 1/2 reach in what we are looking at here, and some plug types have what are called "extended tips" In "old Champion talk" just as EG, a J-8 is a 3/8 reach and 8 is the arbitrarily relative heat range. J-8J is extended tip. An "L" plug is 1/2 reach, "N" is 3/4 reach. I think there were a few "H" plugs which were 7/16 reach, oddball.

In other words you need to backtrack and be certain you have the correct plugs. I would think one severely carboned might impact an electrode, as well

EDIT a quick search says NGK BUHW, and they are 1/2" reach

 

[WinnerCougar74]

If the plug was too long it does not seem like it would have run fine for most of the day but get smashed on the way back to the dock during an extended run at around 4000 rpms. None of the other plugs got smashed at all. There does not seem to be any slop in the piston, rod or crank. That cylinder is very tight. When you rock it back and forth there is no delay at TDC indicating zero wear/slop. So no one thinks it could be minor detonation?

 

[WinnerCougar74]

How's this for a possible explanation. A piece of carbon deposit came loose and got between the plug and the piston. Now that seems likely.

 

[racerone]

Are you putting the correct plugs in ?